JP6340140B2 - Optical communication system and optical communication method - Google Patents

Description

The present invention relates to digital RoF transmission (Radio over Fiber) technology.
This application claims priority based on Japanese Patent Application No. 2015-101840 for which it applied to Japan on May 19, 2015, and uses the content here.

  Conventionally, in a cellular system, in order to improve the degree of freedom of cell configuration, the function of a base station apparatus is defined as a signal processing unit (hereinafter referred to as “BBU” (Base Band Unit)) and an RF unit (hereinafter referred to as “RRH” (Remote Radio Head)))), and BBU and RRH are considered to be physically separated. In such a configuration, a radio signal transmitted between BBU and RRH is transmitted by the RoF technology. The RoF technology can be roughly classified into an analog RoF technology and a digital RoF technology according to an optical transmission method. In recent years, digital RoF technology with excellent transmission quality has been actively studied, and specifications are being developed under a standardization organization such as CPRI (Common Public Radio Interface) (see, for example, Non-Patent Document 1). In addition, a coaxial cable, an optical fiber, or the like is used as a connection medium between BBU and RRH. In particular, a transmission distance can be increased by connecting BBU and RRH with an optical fiber.

Hereinafter, digital RoF transmission will be described.
In describing digital RoF transmission, the following language is defined.
The downlink represents a communication path for radio waves transmitted from the BBU to the wireless terminal connected to the RRH via the RRH.
The uplink represents a communication path of radio waves transmitted from the wireless terminal connected to the RRH to the BBU via the RRH.

In the downlink of digital RoF transmission, the following processing is performed. The BBU creates a digital signal for each I-axis and Q-axis of a radio signal (hereinafter referred to as “IQ data”), converts the created IQ data into an optical signal, and converts the optical signal through an optical fiber. To RRH. The RRH converts the received optical signal into a radio signal and transmits the converted radio signal to the radio terminal.
In the uplink of digital RoF transmission, the following processing is performed. The RRH receives a radio signal transmitted from a radio terminal, converts the received radio signal into an optical signal, and transmits the converted optical signal to the BBU via an optical fiber. The BBU demodulates the signal by converting the received optical signal into IQ data.

FIG. 15 is a schematic block diagram illustrating a functional configuration of the RRH 500 at the time of digital RoF transmission.
The RRH 500 includes an antenna 501, a transmission / reception switching unit 502, an amplifier 503, a down-conversion unit 504, an A / D conversion unit (Analog / Digital) 505, a baseband filter unit 506, a framing unit 507, and an E / O (Electric / Optic). A conversion unit 508, an O / E (Optic / Electric) conversion unit 509, a deframe unit 510, a baseband filter unit 511, a D / A conversion unit (Digital / Analog) 512, an up-conversion unit 513, and an amplifier 514 are provided.

  The antenna 501 transmits and receives radio signals. The transmission / reception switching unit 502 performs transmission / reception switching of the antenna 501. The amplifier 503 amplifies the signal power of the received radio signal to a level that allows signal processing. The down-conversion unit 504 down-converts the amplified radio signal to baseband. The A / D conversion unit 505 converts the down-converted radio signal (analog signal) into IQ data that is a digital signal. The baseband filter unit 506 performs a filtering process on the IQ data. The framing unit 507 forms a frame by multiplexing the IQ data after the filtering process and the control signal. The E / O conversion unit 508 converts a framed signal (hereinafter referred to as “frame signal”) (electrical signal) into an optical signal, and transmits the converted optical signal to the BBU via the optical fiber 550. To do.

  The O / E converter 509 converts an optical signal received via the optical fiber 550 into a frame signal (electric signal). The deframing unit 510 extracts a control signal and IQ data from the frame signal. The baseband filter unit 511 performs a filtering process on the IQ data. The D / A converter 512 converts the IQ data after the filtering process into an analog signal. The up-conversion unit 513 up-converts the analog signal. The amplifier 514 amplifies the power of the analog signal to a predetermined transmission power.

FIG. 16 is a schematic block diagram showing a functional configuration of the BBU 600 at the time of digital RoF transmission.
The BBU 600 includes an O / E converter 601, a deframer 602, a modem unit 603, a framing unit 604, and an E / O converter 605.
The O / E converter 601 converts an optical signal received via the optical fiber 650 into a frame signal (electric signal). The deframing unit 602 extracts a control signal and IQ data from the frame signal. The modem unit 603 restores the radio signal by demodulating the IQ data. Further, the modem unit 603 generates IQ data by modulating the radio signal. The framing unit 604 forms a frame by multiplexing the IQ data and the control signal. The E / O conversion unit 605 converts the frame signal (electric signal) into an optical signal, and transmits the converted optical signal to the RRH 500 via the optical fiber 650.

  Digital RoF transmission requires a very wide band in the optical fiber section. For example, in an LTE (Long Term Evolution) system, a radio signal of 2 × 2 MIMO (Multiple-Input and Multiple-Output) with a system bandwidth of 20 MHz is a maximum of 150 Mbps in a radio section. However, in order to transmit this wireless signal with a 15-bit quantization bit rate, a CPRI link of option 3 (2.4576 Gbps) or higher is required. Therefore, in order to effectively use the optical band, application of a compression technique to digital RoF transmission is being studied. The compression techniques are roughly classified into lossy compression and lossless compression. Examples of lossy compression include a reduction in sampling frequency and a reduction in the number of quantization bits. Lossless compression includes the combined use of linear predictive coding and entropy coding. For example, when the transmission speed of the wireless section is increased, the required transmission band of the optical section increases, but if the required transmission band of the optical section is reduced by compression technology, the wireless section can be increased without changing the optical transceiver. It becomes possible to respond. For example, Non-Patent Document 2 describes MPEG-4 ALS (Moving Picture Experts Group-4 Audio Lossless Coding) which is one of lossless compression techniques.

FIG. 17 is a schematic block diagram showing a functional configuration of the RRH 500a when a compression technique is introduced during multiplex transmission.
The RRH 500a includes an antenna 501, a transmission / reception switching unit 502, an amplifier 503, a down-conversion unit 504, an A / D conversion unit 505, a baseband filter unit 506, a compression unit 701, a framing unit 507a, an E / O conversion unit 508, an O / O An E conversion unit 509, a deframing unit 510, a decompression unit 702, a baseband filter unit 511a, a D / A conversion unit 512, an up-conversion unit 513, and an amplifier 514 are provided.
The compression unit 701 compresses the IQ data after the filtering process. The framing unit 507a forms a frame by multiplexing the compressed IQ data and the control signal. The decompression unit 702 restores the IQ data by decompressing the compressed IQ data. The baseband filter unit 511a performs a filtering process on the restored IQ data.

FIG. 18 is a schematic block diagram showing the functional configuration of the BBU 600a when a compression technique is introduced during multiplex transmission.
The BBU 600 a includes an O / E converter 601, a deframer 602, a decompressor 801, a modem 603 a, a compressor 802, a framer 604 a, and an E / O converter 605.
The decompression unit 801 restores IQ data by decompressing the compressed IQ data. The modem 603a reconstructs the radio signal by demodulating the reconstructed IQ data. Also, the modem unit 603a generates IQ data by modulating the radio signal. The compression unit 802 compresses IQ data. The framing unit 604a forms a frame by multiplexing the compressed IQ data and the control signal.

  Some compression techniques perform compression processing and decompression processing for each predetermined number of samples. In the following description, a unit for performing compression processing is described as a frame, and a predetermined number of samples is described as a frame size. For example, in a compression technique using linear predictive coding, a value obtained by multiplying several sample points past a certain sample point by a coefficient and adding the multiplication result is used as a predicted value, and an error between the predicted value and a certain sample point. Is output. If the prediction accuracy is high, the amplitude value of the error signal is close to zero. Therefore, if the amplitude value having a higher appearance probability is transmitted with a smaller number of bits by entropy coding, the required bandwidth of the optical section can be reduced. The coefficient is determined for each frame, and is calculated so as to reduce the prediction error with respect to the IQ data of each frame.

Next, the LTE radio signal will be described.
In the downlink in LTE, OFDM (Orthogonal Frequency Division Multiplexing) is used. As a time waveform, a signal obtained by adding a cyclic prefix to a signal after IFFT (Inverse Fast Fourier Transform) of a predetermined size is periodically output. On the other hand, in the uplink in LTE, DFT-S-OFDM (Discrete Fourier Transform-Spread OFDM) is used. Also in this case, as in the case of OFDM, a signal having a cyclic prefix added to a signal after IFFT having a predetermined size is periodically output as a time waveform. In the following description, a signal after cyclic IFFT added with a cyclic prefix is referred to as an OFDM symbol, and is used without distinction between downlink and uplink.

  In LTE, a normal cyclic prefix and an extended cyclic prefix are defined. The normal cyclic prefix is shorter in size and more efficient in frequency utilization than the extended cyclic prefix. Therefore, since a normal cyclic prefix is normally used, the following description will be made taking a normal cyclic prefix as an example. FIG. 19 shows the structure of a time slot in LTE. In the example shown in FIG. 19, seven OFDM symbols are arranged in a 0.5 ms interval. When the system bandwidth is 20 MHz, the IFFT size is 2048, the cyclic prefix (CP1) of the first OFDM symbol is 160 points, and the cyclic prefix (CP2) of the second to seventh OFDM symbols is 144 points. Therefore, the OFDM symbol length is 2208 points for the first OFDM symbol and 2192 points for the second to seventh OFDM symbols. Thus, not all OFDM symbol lengths are the same. Non-Patent Document 3 describes a configuration related to an LTE frame.

FIG. 20 is a diagram showing a compression rate for each frame when MPEG4-ALS is applied to I component data of a radio signal.
In FIG. 20, the frame number represents the order of frames subjected to compression processing. The compression rate is a ratio of the data amount after compression to the original data amount. The radio signal is OFDM modulation, subcarrier spacing is 15 kHz, the number of subcarriers is 1200, 256QAM (Quadrature Amplitude Modulation) modulation, and the cyclic prefix is 160 samples (first OFDM symbol) or 144 samples (second OFDM symbol to seventh OFDM symbol). . That is, it is assumed that all radio bands are used for data transmission in an LTE downlink system with a system bandwidth of 20 MHz. The frame size was 548.

  In FIG. 20, (a) represents the compression rate when only the first OFDM symbol is included in the frame. (B) represents the compression rate when the first OFDM symbol and the second OFDM symbol are included in the frame. (C) represents the compression rate when only the second OFDM symbol is included in the frame. (D) represents the compression rate when the second OFDM symbol and the third OFDM symbol are included in the frame. (E) represents the compression rate when only the third OFDM symbol is included in the frame. (F) represents the compression rate when the third OFDM symbol and the fourth OFDM symbol are included in the frame. (G) represents the compression rate when only the fourth OFDM symbol is included in the frame. (H) represents the compression rate when the fourth OFDM symbol and the fifth OFDM symbol are included in the frame. (I) represents the compression rate when only the fifth OFDM symbol is included in the frame. (J) represents the compression rate when the fifth OFDM symbol and the sixth OFDM symbol are included in the frame.

CPRI, “CPRI Specification V6.0,” Aug. 2013, http://www.cpri.info/spec.html Yu Kamamoto, Takehiro Moriya, Noboru Harada, Csaba Kos, “High-performance lossless audio coding MPEG-4 ALS,” NTT Technical Journal, Feb. 2008 3 Erik Dahlman (author), Takeshi Hattori (supervised), “All 3G Evolution LTE Mobile Broadband Technology”, Dec. 2009, pp. 356, 425

  As shown in FIG. 20, the compression rate when the compression process is performed without including a plurality of types of OFDM symbols is less than 0.7, but the compression process is performed with a plurality of types of OFDM symbols. In all cases, the compression ratio exceeds 0.7. That is, when the compression process is performed including a plurality of types of OFDM symbols, the compression rate is deteriorated as compared with the case where the compression process is performed within one type of OFDM symbol. This is thought to be due to the fact that the frequency accuracy is different for each OFDM symbol and the nature of the signal is different, so that the accuracy of prediction is reduced. As described above, the conventional technique has a problem in that the compression rate is deteriorated by performing compression processing including a plurality of types of OFDM symbols.

  In view of the above circumstances, an object of the present invention is to provide a technique capable of reducing deterioration in compression rate.

  One aspect of the present invention includes a signal processing device having a function of a divided base station and a wireless device, and has a predetermined size by digital RoF (Radio over Fiber) transmission between the signal processing device and the wireless device. An optical communication system for transmitting a periodic symbol sequence in which a cyclic prefix is added to a signal after IFFT (Inverse Fast Fourier Transform), wherein each of the signal processing device and the wireless device includes a transmitter and a receiver The transmitter is configured to acquire symbol information related to a start position of the symbol series and a length of each symbol constituting the symbol series, and to perform compression processing based on the acquired symbol information A compression size determination unit that determines a compression size for each symbol, and compresses the symbol series in units of the determined compression size. A compression unit that performs the decompression process of the symbol sequence, and a decompression size determination unit that determines a decompression size for each symbol to be subjected to the decompression process of the symbol sequence, and the symbol series in units of the determined decompression size 1 is an optical communication system comprising:

  In the above optical communication system, the transmission unit further includes a compression rate measurement unit that measures a compression rate for each symbol, and the compression size determination unit has a predetermined statistical value of the measured compression rate being minimized. The position of the symbol may be acquired as the start position, and the compression size may be determined using the acquired start position and information regarding the length of each symbol.

  In the above optical communication system, the transmission unit may further include a symbol information estimation unit that estimates the start position based on downlink or uplink IQ data.

  One embodiment of the present invention includes a signal processing device and a wireless device each having a divided base station function, and each of the signal processing device and the wireless device includes a transmission unit and a reception unit, An optical communication system for transmitting a periodic symbol sequence in which a cyclic prefix is added to a signal after IFFT (Inverse Fast Fourier Transform) of a predetermined size by digital RoF (Radio over Fiber) transmission with the wireless device In this optical communication method, the transmitting unit acquires symbol information related to a start position of the symbol series and a length of each symbol constituting the symbol series, and performs compression processing based on the acquired symbol information. A compression size determination step for determining a compression size for each symbol to be performed, and the transmission unit determines the determined A compression step of compressing the symbol sequence in units of compression size; an expansion size determination step in which the reception unit determines an expansion size for each symbol to be subjected to the expansion processing of the symbol sequence; and the reception unit, A decompression step of decompressing the symbol sequence in units of the determined decompression size.

  According to the present invention, it is possible to reduce the deterioration of the compression rate.

It is a schematic block diagram showing the function structure of RRH100 in 1st Embodiment. It is a schematic block diagram showing the function structure of BBU200 in 1st Embodiment. It is a figure showing the operation example of a compression process size determination part and an expansion | extension process size determination part. It is a flowchart which shows the flow of a process in the uplink of RRH100 in 1st Embodiment. It is a flowchart which shows the flow of the process in the uplink of BBU200 in 1st Embodiment. It is a flowchart which shows the flow of the process in the downlink of RRH100 in 1st Embodiment. It is a flowchart which shows the flow of the process in the downlink of BBU200 in 1st Embodiment. It is a schematic block diagram showing the function structure of RRH100a in 2nd Embodiment. It is a schematic block diagram showing the functional structure of BBU200a in 2nd Embodiment. It is a flowchart which shows the flow of a process in the uplink of RRH100a in 2nd Embodiment. It is a flowchart which shows the flow of an OFDM symbol start position information acquisition process. It is a schematic block diagram showing the function structure of RRH100b in 3rd Embodiment. It is a schematic block diagram showing the functional structure of BBU200b in 3rd Embodiment. It is a flowchart which shows the flow of a process in the uplink of RRH100b in 3rd Embodiment. It is a schematic block diagram showing the function structure of RRH500 at the time of digital RoF transmission. It is a schematic block diagram showing the functional structure of BBU600 at the time of digital RoF transmission. It is a schematic block diagram showing the function structure of RRH500a at the time of introducing a compression technique at the time of multiplex transmission. It is a schematic block diagram showing the functional structure of BBU600a at the time of introducing a compression technique at the time of multiplex transmission. It is a figure which shows the structure of the time slot in LTE. It is a figure which shows the compression rate for every flame | frame at the time of applying MPEG4-ALS with respect to the data of I component of a radio signal.

Embodiments of the present invention will be described below with reference to the drawings.
[Overview]
In the present invention, a symbol sequence in which an RRH and a BBU in an optical communication system including an RRH (wireless device) having a divided base station function and a BBU (signal processing device) are composed of a plurality of OFDM symbols (symbols). Information about the start position of each OFDM symbol and the length of each OFDM symbol (hereinafter referred to as “OFDM symbol information”). Then, the RRH and BBU determine the frame size for each OFDM symbol to be subjected to compression processing based on the acquired OFDM symbol information, and perform the compression processing with the determined frame size.
Hereinafter, a plurality of embodiments (first embodiment to third embodiment) will be specifically described as examples.

[First Embodiment]
In the first embodiment, the RRH and BBU acquire OFDM symbol information (symbol information), determine the frame size for each OFDM symbol to be subjected to compression processing based on the acquired OFDM symbol information, and determine Perform compression processing at the frame size. Moreover, RRH and BBU determine the frame size for each OFDM symbol to be subjected to the decompression process based on the acquired OFDM symbol information, and perform the decompression process with the determined frame size.

FIG. 1 is a schematic block diagram illustrating a functional configuration of the RRH 100 according to the first embodiment. FIG. 2 is a schematic block diagram showing the functional configuration of the BBU 200 in the first embodiment. First, the RRH 100 will be described.
The RRH 100 includes an antenna 101, a transmission / reception switching unit 102, an amplifier 103, a down-conversion unit 104, an A / D conversion unit 105, a baseband filter unit 106, a compression processing size determination unit 107, a compression unit 108, a framing unit 109, an E / D An O conversion unit 110, an O / E conversion unit 111, a deframe conversion unit 112, an expansion processing size determination unit 113, an expansion unit 114, a baseband filter unit 115, a D / A conversion unit 116, an up-conversion unit 117, and an amplifier 118 Prepare.

The antenna 101 transmits / receives a radio signal to / from a radio terminal connected to the RRH 100. The transmission / reception switching unit 102 performs transmission / reception switching of the antenna 101. Note that the transmission / reception switching unit 102 can handle both FDD (Frequency Division Duplex) and TDD (Time Division Duplex). For example, when the BBU-RRH is a CPRI interface, approximately 1/16 of the whole is used for sending control signals and 15/16 is used for sending IQ data, and K28.5 code for establishing a CPRI link as a control signal. Etc. are transmitted.
The amplifier 103 amplifies the signal power of the received radio signal to a level that allows signal processing. The down-conversion unit 104 down-converts the radio signal to baseband. The A / D converter 105 converts the down-converted radio signal (analog signal) into IQ data that is a digital signal. The baseband filter unit 106 performs a filtering process on the IQ data. By this process, an OFDM symbol of a radio signal is generated.

The compression processing size determination unit 107 determines the frame size (hereinafter referred to as “compression size”) of a frame to be subjected to compression processing based on the acquired OFDM symbol information. Here, the frame is a unit for performing compression processing, and is configured by arranging IQ data by a predetermined number of samples in a time waveform, for example.
The compressing unit 108 compresses the OFDM symbol in units of the compression size determined by the compression processing size determining unit 107 for each frame.

The framing unit 109 generates a frame signal by multiplexing the compressed OFDM symbol and the control signal.
The E / O conversion unit 110 converts the frame signal (electrical signal) into an optical signal, and transmits the converted optical signal to the BBU 200 via the optical fiber 150.
The O / E converter 111 converts an optical signal received via the optical fiber 150 into a frame signal (electric signal).

The deframing unit 112 extracts a control signal and a compressed OFDM symbol from the frame signal.
The decompression processing size determination unit 113 determines a frame size (hereinafter referred to as “expansion size”) for performing the decompression processing based on the frame size acquired from the BBU 200. As a method for the decompression processing size determination unit 113 to obtain the decompression size information, the compression unit of the BBU 200 adds the frame size to the beginning of the compressed data as a header and transmits it. The frame size is determined based on the header. A method of obtaining is conceivable. In the case of the LTE system, at least two frame sizes are sufficient, and at this time, the header may be 1 bit. The decompression size and the compression size are the same size.

The decompression unit 114 decompresses the compressed OFDM symbol in units of the decompression size determined by the decompression processing size determination unit 113. Specifically, the decompression unit 114 decompresses the compressed OFDM symbol in units of the determined decompression size to restore it to an OFDM symbol.
The baseband filter unit 115 performs a filtering process on the restored OFDM symbol.
The D / A converter 116 converts the signal after the filtering process into an analog signal.
The up-conversion unit 117 up-converts the analog signal.
The amplifier 118 amplifies the power of the analog signal to a predetermined transmission power.

Next, the BBU 200 will be described.
The BBU 200 includes an O / E conversion unit 201, a deframing unit 202, an expansion processing size determination unit 203, an expansion unit 204, a modulation / demodulation unit 205, a compression processing size determination unit 206, a compression unit 207, a framing unit 208, and an E / O. A conversion unit 209 is provided.
The O / E converter 201 converts an optical signal received via the optical fiber 250 into a frame signal (electric signal). The deframing unit 202 extracts a control signal and a multiplexed signal from the frame signal.

The expansion process size determination unit 203 determines the expansion size based on the frame size acquired from the RRH 100. As a method for the expansion processing size determination unit 203 to acquire the expansion size, a method of acquiring the expansion size by a process similar to that of the expansion processing size determination unit 113 can be considered.
The expansion unit 204 expands the compressed OFDM symbol in units of the expansion size determined by the expansion processing size determination unit 203. Specifically, the decompression unit 204 decompresses the compressed OFDM symbol in units of the determined decompression size to restore it to an OFDM symbol.
The modem unit 205 restores the radio signal by demodulating the restored OFDM symbol. Further, the modem unit 205 outputs the IQ data of the radio signal to the compression processing size determination unit 206 and the compression unit 207.

  The compression processing size determination unit 206 determines the compression size based on the acquired OFDM symbol information. The relationship between the OFDM symbol length and the frame size may be determined in advance. In this case, the compression processing size determining unit 206 determines the compression size based on the OFDM symbol length of the acquired OFDM symbol information and the predetermined frame size. For example, the compression processing size determination unit 206 determines the frame size corresponding to the OFDM symbol length as the compression size.

The compression unit 207 compresses the OFDM symbol in units of compression size determined by the compression processing size determination unit 206 for each frame.
The framing unit 208 generates a frame signal by multiplexing the compressed OFDM symbol and the control signal.
The E / O conversion unit 209 converts the frame signal (electrical signal) into an optical signal, and transmits the converted optical signal to the RRH 100 via the optical fiber 250.

  In the following description, when the compression unit 108 and the compression unit 207 are not particularly distinguished, they are simply referred to as a compression unit. In the following description, the expansion unit 114 and the expansion unit 204 are simply referred to as an expansion unit unless otherwise distinguished. In the following description, the compression processing size determination unit 107 and the compression processing size determination unit 206 will be simply referred to as a compression processing size determination unit unless otherwise distinguished. In the following description, the expansion process size determination unit 113 and the expansion process size determination unit 203 are simply referred to as an expansion process size determination unit unless otherwise distinguished.

  The compression unit performs compression processing according to the determined compression size. Further, the decompression unit performs decompression processing according to the determined decompression size. For example, when linear predictive coding is performed in the compression unit, the number of samples used for obtaining the coefficient is changed when the frame size (compression size) is changed. The compression unit / decompression unit may be provided with a compression / decompression circuit for each frame size, and which one of them is used may be switched according to the notified frame size.

  It is conceivable that the compression processing size determination unit 206 acquires the OFDM symbol information from the modulation / demodulation unit 205 of the BBU 200. In this case, it is necessary to notify the OFDM symbol information to the compression processing size determination unit 107 of the RRH 100. Therefore, when the BBU 200-RRH 100 is a CPRI interface, OFDM symbol information can be transmitted using reserved bits of the CPRI control signal. The compression processing size determination unit 107 acquires the OFDM symbol information by receiving the OFDM symbol information from the BBU 200.

  For example, in the case of a TDD LTE system, downlink and uplink communications are interchanged with a minimum period of 1 ms. Therefore, if the OFDM symbol start position / OFDM symbol length information of the downlink 0.5 ms period is known, the uplink OFDM symbol start position is known. -OFDM symbol length information can also be estimated. Usually, since the OFDM symbol length is fixed for each system, the OFDM symbol length information may be stored in advance in the compression processing size determination unit. For example, in an LTE radio system, if the start position of an OFDM symbol having a CP (cyclic prefix) length of 160 is known, the start position of the subsequent OFDM symbol and the OFDM symbol length can be known. Therefore, it is only necessary to obtain start position information of an OFDM symbol having a CP length of 160. Further, since the OFDM signal is continuously output, it is only necessary to know the start position of the OFDM symbol once, and it is not necessary to periodically acquire the start position information.

FIG. 3 is a diagram illustrating an operation example of the compression processing size determination unit and the decompression processing size determination unit.
In the example shown in FIG. 3, a plurality of OFDM symbols are shown. (A) represents the first OFDM symbol, (b) represents the second OFDM symbol, and (c) represents the third OFDM symbol. Here, if the number of frames in the jth OFDM symbol is Aj and the frame size of each frame is a _{i, j} (1 ≦ i ≦ Aj), (a) to (c) are as shown in FIG. Can be represented. Thus, the compression processing size determining unit determines the frame size a _{i, j} for each OFDM symbol so as not to include a plurality of OFDM symbols. In the LTE system, since the OFDM symbol lengths of the second to seventh OFDM symbols are equal, a _{i, 2} = a _{i, 3} = a _{i, 4} = a _{i, 5} = a _{i, 6} = a _{i, 7} . In this case, the compression processing size determination units 107 and 206 may determine two frame sizes, one for the first OFDM symbol and one for the second to seventh OFDM symbols. In addition, a _{1, j} = a _{2, j} =... = A _{Aj, j} may be simply used. Further, the decompression processing size determination units 113 and 203 may similarly determine the frame size (expansion size).

FIG. 4 is a flowchart illustrating a process flow in the uplink of the RRH 100 according to the first embodiment.
The antenna 101 receives a radio signal (step S101). The antenna 101 outputs the received radio signal to the amplifier 103 via the transmission / reception switching unit 102. The amplifier 103 amplifies the signal power of the radio signal to a level that allows signal processing (step S102). The down-conversion unit 104 down-converts the radio signal to baseband (step S103). Thereafter, the A / D conversion unit 105 converts the down-converted radio signal into IQ data that is a digital signal (step S104). The baseband filter unit 106 performs a filtering process on the IQ data (step S105).

  The compression processing size determination unit 107 determines the compression size based on the OFDM symbol information acquired from the BBU 200 (step S106). The compression unit 108 compresses the OFDM symbol in units of the compression size determined by the compression processing size determination unit 107 (step S107). The framing unit 109 generates a frame signal by multiplexing the compressed OFDM symbol and the control signal (step S108). The E / O conversion unit 110 converts the frame signal into an optical signal (step S109). Then, the E / O conversion unit 110 transmits an optical signal to the BBU 200 via the optical fiber 150 (step S110).

FIG. 5 is a flowchart showing a processing flow in the uplink of the BBU 200 in the first embodiment.
The O / E converter 201 converts an optical signal received via the optical fiber 250 into a frame signal (electric signal) (step S201). The O / E converter 201 outputs the frame signal to the deframer 202. The deframing unit 202 extracts a control signal and a compressed OFDM symbol from the frame signal (step S202). The decompression processing size determination unit 203 determines the decompression size based on the frame size notified from the RRH 100 (step S203).

  The decompression unit 204 decompresses the compressed OFDM symbol in units of the decompression size determined by the decompression processing size determination unit 203 and restores it to an OFDM symbol (step S204). The modem unit 205 restores the radio signal by demodulating the restored OFDM symbol (step S205). The modem unit 205 receives the restored radio signal (step S206). Note that the reception in the process of step S206 means that the modem unit 205 acquires a radio signal to demodulate the OFDM symbol.

FIG. 6 is a flowchart illustrating a processing flow in the downlink of the RRH 100 according to the first embodiment.
The O / E converter 111 converts the optical signal received via the optical fiber 150 into a frame signal (electric signal) (step S301). The deframing unit 112 extracts the control signal and the compressed OFDM symbol from the frame signal (step S302). The expansion processing size determination unit 113 determines the expansion size based on the frame size notified from the BBU 200. (Step S303). The decompression unit 114 decompresses the compressed OFDM symbol in units of the frame size determined by the decompression processing size determination unit 113 and restores it to an OFDM symbol (step S304).

  The baseband filter unit 115 performs a filtering process on the restored OFDM symbol (step S305). The D / A conversion unit 116 converts the filtered signal into an analog signal (step S306). The up-conversion unit 117 up-converts the analog signal (step S307). The amplifier 118 amplifies the power of the analog signal to the determined transmission power (step S308). The antenna 101 transmits an analog signal to the wireless terminal connected to the RRH 100 (step S309).

FIG. 7 is a flowchart showing a process flow in the downlink of the BBU 200 in the first embodiment.
The modem unit 205 outputs the IQ data of the radio signal to the compression processing size determination unit 206 and the compression unit 207 (step S401). The compression processing size determination unit 206 determines the compression size based on the acquired OFDM symbol information (step S402). The compression unit 207 compresses the OFDM symbol in units of the compression size determined by the compression processing size determination unit 206 (step S403). The framing unit 208 generates a frame signal by multiplexing the compressed OFDM symbol and the control signal (step S404). The E / O conversion unit 209 converts the frame signal (electric signal) into an optical signal (step S405). The E / O conversion unit 209 transmits the optical signal to the RRH 100 via the optical fiber 250 (step S406).

According to the RRH 100 and the BBU 200 configured as described above, it is possible to reduce the deterioration of the compression rate. Hereinafter, this effect will be described in detail.
The RRH 100 and the BBU 200 determine a compression size for compressing the radio signal based on the acquired OFDM symbol information. By this process, the RRH 100 and the BBU 200 determine the frame size so as not to include OFDM symbols having different frequency characteristics when performing the compression process. Then, the RRH 100 and the BBU 200 perform compression processing in units of the determined compression size. Therefore, it becomes possible to reduce deterioration of the compression rate as a whole. In addition, since the deterioration of the compression rate is reduced, the transmission band can be used effectively.

[Second Embodiment]
In the second embodiment, the RRH and BBU acquire information on the start position of the OFDM symbol based on the compression rate for each OFDM symbol. Then, the RRH and BBU determine the frame size in the OFDM symbol to be compressed based on the acquired information on the start position of the OFDM symbol and the information on the OFDM symbol length, and perform the compression processing with the determined frame size. .

FIG. 8 is a schematic block diagram illustrating a functional configuration of the RRH 100a according to the second embodiment. FIG. 9 is a schematic block diagram showing the functional configuration of the BBU 200a in the second embodiment. First, the RRH 100a will be described.
The RRH 100a includes an antenna 101, a transmission / reception switching unit 102, an amplifier 103, a down-conversion unit 104, an A / D conversion unit 105, a baseband filter unit 106, a compression processing size determination unit 107a, a compression unit 108, a framing unit 109, an E / O converter 110, O / E converter 111, deframer 112, decompression processing size determiner 113, decompressor 114, baseband filter 115, D / A converter 116, upconverter 117, amplifier 118, A compression rate measurement unit 119 is provided.

  The RRH 100a is different from the RRH 100 in that it includes a compression processing size determination unit 107a instead of the compression processing size determination unit 107 and a compression rate measurement unit 119. The RRH 100a is the same as the RRH 100 in other configurations. Therefore, description of the entire RRH 100a is omitted, and the compression processing size determination unit 107a and the compression rate measurement unit 119 will be described.

The compression rate measurement unit 119 measures the compression rate for each OFDM symbol from the output of the compression unit 108.
The compression processing size determination unit 107a determines the compression size based on the compression rate measured by the compression rate measurement unit 119. Specifically, the compression processing size determination unit 107a acquires a position where the average value or the maximum value of the measured compression rate is minimum as the OFDM symbol start position. Then, the compression processing size determination unit 107a determines the compression size from the acquired OFDM symbol start position information and OFDM symbol length information so as not to span OFDM symbols having different frequency characteristics. In the second embodiment, the compression processing size determination unit 107a needs to acquire the information of the OFDM symbol length by the method of the first embodiment or store it in advance.

Next, the BBU 200a will be described.
The BBU 200a includes an O / E conversion unit 201, a deframing unit 202, an expansion processing size determination unit 203, an expansion unit 204, a modem unit 205, a compression processing size determination unit 206a, a compression unit 207, a framing unit 208, and an E / O. A conversion unit 209 and a compression rate measurement unit 210 are provided. The BBU 200a is different from the BBU 200 in that a compression processing size determination unit 206a is provided instead of the compression processing size determination unit 206, and a compression rate measurement unit 210 is newly provided. The BBU 200a is the same as the BBU 200 in other configurations. Therefore, description of the entire BBU 200a is omitted, and the compression processing size determination unit 206a and the compression rate measurement unit 210 will be described. The processing of the compression processing size determination unit 206a and the compression rate measurement unit 210 is the same as the processing of the compression processing size determination unit 107a and the compression rate measurement unit 119.

FIG. 10 is a flowchart illustrating a processing flow in the uplink of the RRH 100a in the second embodiment. In addition, about the process similar to FIG. 4, the code | symbol similar to FIG. 4 is attached | subjected in FIG. 10, and description is abbreviate | omitted.
The RRH 100a performs an OFDM symbol start position information acquisition process (step S501). The OFDM symbol start position information acquisition process will be described later. Then, the compression processing size determination unit 107a determines the compression size from the OFDM symbol start position information and the OFDM symbol length information acquired in step S501 so as not to straddle OFDM symbols having different frequency characteristics. (Step S502). Thereafter, the processing after step S107 is executed.

FIG. 11 is a flowchart showing a flow of OFDM symbol start position information acquisition processing. In FIG. 11, a case of LTE with a system bandwidth of 20 MHz will be described as an example. In LTE with a system bandwidth of 20 MHz, OFDM symbols are transmitted with a period of 0.5 ms (15360 samples). For this reason, the compression processing size determination unit 107a only needs to be able to acquire information on the head position of 15360 samples.
First, the compression processing size determination unit 107a sets i = 0, i _a = 0, and the provisional minimum value to ∞ as initial values (step S601). Here, i represents the frame number, i _a represents the estimated value of the OFDM symbol start position, and the provisional minimum value represents the minimum value of the compression rate. The compression rate measuring unit 119 measures the compression rate (step S602). The compression processing size determination unit 107a determines whether or not the measured compression rate is smaller than the provisional minimum value (step S603). If the measured compression ratio is smaller than the temporary minimum value (step S603-YES), the compression processing size determination unit 107a sets the i _a to the value of i, the temporary minimum value is set to the measured compression ratio (Step S604). Thereafter, the compression processing size determination unit 107a adds 1 to the value of i (step S605). The compression processing size determination unit 107a determines whether or not the value of i is equal to or greater than a preset i _max (step S606). Here, in the case of LTE with a system bandwidth of 20 MHz, i _max is 15360.

If the value of i is _{i max} or more set in advance (Step S606-YES), the best OFDM compression rate characteristic from among the estimated values _{i a} compression processing size determination unit 107a every OFDM symbol start position estimates _{i a} symbol start position is obtained as the start position information of the OFDM symbol (step S607). Here, the start position of the OFDM symbol when the compression rate characteristic is the best is the position where the average value or the maximum value of the compression rate becomes the minimum.

Further, in the process of step S606, when the value of i is not equal to or greater than the preset i _max (step S606-NO), the RRH 100a repeatedly executes the processes after step S602.
Also, in the process of step S603, when the measured compression rate is not smaller than the provisional minimum value (step S603-NO), the compression process size determination unit 107a adds 1 to the value of i (step S605).

According to the RRH 100a and the BBU 200a configured as described above, it is possible to obtain the same effect as in the first embodiment.
Further, the RRH 100a and the BBU 200a acquire information on the start position of the OFDM symbol from the compression rate of the compression processing performed by the compression unit 108 and the compression unit 207. Then, the RRH 100a and the BBU 200a determine the compression size based on the acquired OFDM symbol start position information and OFDM symbol length information. Therefore, more accurate compression becomes possible. In addition, correction can be performed even if the start position of the OFDM symbol is deviated from the expected due to the influence of the radio propagation environment, processing delay of BBU / RRH, fiber delay between BBU 200a and RRH 100a, and the like.

  The compression rate measurement unit 119 and the compression rate measurement unit 210 may reduce the amount of calculation using an analysis result performed by the compression unit instead of the compression rate measurement. Specifically, the compression rate measuring unit 119 and the compression rate measuring unit 210 estimate the information amount for each frame, that is, the compression rate, based on the autocorrelation coefficient and the PARCOR (Partial Auto-Correlation) coefficient obtained during the linear prediction analysis. This processing can reduce the amount of processing required for entropy coding.

[Third Embodiment]
In the third embodiment, the RRH and the BBU acquire OFDM symbol information from downlink or uplink IQ data. Then, the RRH and the BBU determine the frame size in the OFDM symbol to be compressed based on the acquired OFDM symbol information, and perform the compression process with the determined frame size.

FIG. 12 is a schematic block diagram illustrating a functional configuration of the RRH 100b according to the third embodiment. FIG. 13 is a schematic block diagram showing the functional configuration of the BBU 200b in the third embodiment. First, the RRH 100b will be described.
The RRH 100b includes an antenna 101, a transmission / reception switching unit 102, an amplifier 103, a down-conversion unit 104, an A / D conversion unit 105, a baseband filter unit 106, a compression processing size determination unit 107b, a compression unit 108, a framing unit 109, an E / D O converter 110, O / E converter 111, deframer 112, decompression processing size determiner 113, decompressor 114, baseband filter 115, D / A converter 116, upconverter 117, amplifier 118, An OFDM symbol information estimation unit 120 is provided.

  The RRH 100b is different from the RRH 100 in that the RRH 100b includes a compression processing size determination unit 107b instead of the compression processing size determination unit 107, and further includes an OFDM symbol information estimation unit 120. The RRH 100b is the same as the RRH 100 in other configurations. Therefore, description of the entire RRH 100b is omitted, and the compression processing size determination unit 107b and the OFDM symbol information estimation unit 120 will be described.

The OFDM symbol information estimation unit 120 estimates OFDM symbol start position and OFDM symbol length information from the uplink signal.
The compression processing size determination unit 107b acquires the OFDM symbol information estimated by the OFDM symbol information estimation unit 120. Then, the compression processing size determination unit 107b determines the compression size based on the acquired OFDM symbol information.

Next, the BBU 200b will be described.
The BBU 200b includes an O / E conversion unit 201, a deframe unit 202, a decompression processing size determination unit 203, a decompression unit 204, a modulation / demodulation unit 205, a compression processing size determination unit 206b, a compression unit 207, a framing unit 208, and an E / O. A conversion unit 209 and an OFDM symbol information estimation unit 211 are provided. The BBU 200b differs from the BBU 200 in that it includes a compression processing size determination unit 206b instead of the compression processing size determination unit 206, and newly includes an OFDM symbol information estimation unit 211. The BBU 200b is the same as the BBU 200 in other configurations. Therefore, description of the entire BBU 200b is omitted, and the compression processing size determination unit 206b and the OFDM symbol information estimation unit 211 will be described.

The OFDM symbol information estimation unit 211 estimates information on the OFDM symbol start position and OFDM symbol length from the downlink signal.
The compression processing size determination unit 206b acquires the OFDM symbol information estimated by the OFDM symbol information estimation unit 211. Then, the compression processing size determination unit 206b determines the compression size based on the acquired OFDM symbol information.

FIG. 14 is a flowchart illustrating a process flow in the uplink of the RRH 100b according to the third embodiment. In addition, about the process similar to FIG. 4, the code | symbol similar to FIG. 4 is attached | subjected in FIG. 14, and description is abbreviate | omitted.
The OFDM symbol information estimation unit 120 estimates information on the OFDM symbol start position and OFDM symbol length from the uplink signal (step S801). As a method for estimating the OFDM symbol start position and OFDM symbol length information, there is a method of measuring EVM (Error Vector Magnitude) by performing FFT conversion on IQ data. At this time, the OFDM symbol information estimation unit 120 shifts the FFT window one point at a time, and the OFDM symbol start position and cyclic prefix length (OFDM symbol length) information at the position and period at which the EVM after the FFT is minimized. Is estimated. Alternatively, the OFDM symbol information estimation unit 120 may estimate the OFDM symbol start position and cyclic prefix length (OFDM symbol length) information from the autocorrelation of the uplink signal using the cyclic prefix periodicity. The OFDM symbol length information may be acquired by the method of the first embodiment, or may be stored in advance in the OFDM symbol information estimation unit 120.

According to the RRH 100b and the BBU 200b configured as described above, it is possible to obtain the same effect as that of the first embodiment.
Further, the RRH 100b and the BBU 200b can perform correction even if the start position of the OFDM symbol is deviated from the expected due to the influence of the radio propagation environment, the processing delay of the BBU / RRH, the fiber delay between the BBU 200b and the RRH 100b, etc. . Further, the RRH 100b and the BBU 200b can perform the OFDM symbol start position and the OFDM symbol from the downlink signal and the uplink signal, respectively, without requiring additional information for estimating the OFDM symbol start position and OFDM symbol length information. Long information can be estimated.

  In addition, the program for executing each process of RRH100, RRH100a, RRH100b, BBU200, BBU200a, and BBU200b of the present invention is recorded on a computer-readable recording medium, and the program recorded on the recording medium is read into a computer system. The above-described various processes relating to the processes of the RRH 100, the RRH 100a, the RRH 100b, the BBU 200, the BBU 200a, and the BBU 200b may be performed. The “computer system” herein may include an OS (Operating System) and hardware such as peripheral devices. Further, the “computer system” includes a homepage providing environment (or display environment) if a WWW (World Wide Web) system is used. The “computer-readable recording medium” means a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), a writable nonvolatile memory such as a flash memory, and a portable medium such as a CD (Compact Disc) -ROM. A storage device such as a hard disk built in a computer system.

  Further, the “computer-readable recording medium” means a volatile memory (for example, DRAM (Dynamic DRAM) in a computer system that becomes a server or a client when a program is transmitted through a network such as the Internet or a communication line such as a telephone line. Random Access Memory)), etc., which hold programs for a certain period of time. The program may be transmitted from a computer system storing the program in a storage device or the like to another computer system via a transmission medium or by a transmission wave in the transmission medium. Here, the “transmission medium” for transmitting the program refers to a medium having a function of transmitting information, such as a network (communication network) such as the Internet or a communication line (communication line) such as a telephone line. The program may be for realizing a part of the functions described above. Further, the program may be a so-called difference file (difference program) that can realize the above-described functions in combination with a program already recorded in the computer system.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes designs and the like that do not depart from the gist of the present invention.

  The present invention is applicable to, for example, digital RoF transmission. According to the present invention, it is possible to reduce deterioration of the compression rate.

100, 100a, 100b, 500, 500a ... RRH
200, 200a, 200b, 600, 600a ... BBU
101, 501 ... antennas 102, 502 ... transmission / reception switching units 103, 503 ... amplifiers 104, 504 ... down-conversion units 105, 505 ... A / D conversion units 106, 506 ... baseband filter units (upstream)
107, 107a, 107b ... compression processing size determining unit 108, 701 ... compression unit 109, 507, 507a ... framing unit 110, 508 ... E / O conversion unit 111, 509 ... O / E conversion unit 112, 510 ... deframe Conversion unit 113 ... Decompression processing size determination unit 114, 702 ... Extension unit 115, 511, 511a ... Baseband filter unit (downlink)
116, 512 ... D / A converters 117, 513 ... Up-converter 118, 514 ... Amplifier 119 ... Compression rate measuring unit 120 ... OFDM symbol information estimator 150, 250, 550, 650 ... Optical fibers 201, 601 ... O / E-conversion units 202, 602 ... deframe unit 203 ... decompression processing size determination unit 204,801 ... decompression unit 205, 603, 603a ... modulation / demodulation unit 206, 206a, 206b ... compression processing size determination unit 207, 802 ... compression unit 208 604, 604a, framing unit 209, 605, E / O conversion unit 210, compression rate measuring unit 211, OFDM symbol information estimation unit

Claims (4)

An IFFT (Inverse Fast Fourier Transform) having a predetermined size by digital RoF (Radio over Fiber) transmission between the signal processing device and the wireless device. An optical communication system for transmitting a periodic symbol sequence with a cyclic prefix added to a later signal,
Each of the signal processing device and the wireless device includes a transmission unit and a reception unit,
The transmitter is
The symbol information regarding the start position of the symbol series and the length of each symbol constituting the symbol series is acquired, and the compression size for each symbol to be subjected to compression processing is determined based on the acquired symbol information. A compression size determination unit;
A compression unit that compresses the symbol sequence in units of the determined compression size;
With
The receiver is
An extension size determination unit that determines an extension size for each symbol to be subjected to extension processing of the symbol series;
A decompression unit that decompresses the symbol sequence in units of the determined decompression size;
An optical communication system comprising: The transmission unit further includes a compression rate measurement unit that measures a compression rate for each symbol,
The compression size determination unit acquires, as the start position, the position of the symbol at which a predetermined statistical value of the measured compression rate is minimum, and acquires the acquired start position and information on the length of each symbol. The optical communication system according to claim 1, wherein the compression size is used to determine the compression size.   The optical transmission system according to claim 1, wherein the transmission unit further includes a symbol information estimation unit that estimates the start position based on downlink or uplink IQ data. A signal processing device and a wireless device each having a divided base station function, each of the signal processing device and the wireless device including a transmission unit and a reception unit; and between the signal processing device and the wireless device An optical communication method in an optical communication system for transmitting a periodic symbol sequence in which a cyclic prefix is added to a signal after IFFT (Inverse Fast Fourier Transform) of a predetermined size by digital RoF (Radio over Fiber) transmission. ,
The transmitting unit acquires symbol information related to a start position of the symbol series and a length of each symbol constituting the symbol series, and based on the acquired symbol information, for each symbol to be subjected to compression processing A compression size determination step for determining a compression size;
A compression step in which the transmission unit compresses the symbol sequence in units of the determined compression size;
An extension size determination step in which the receiving unit determines an extension size for each symbol to be subjected to the extension processing of the symbol series;
An expansion step in which the receiving unit expands the symbol sequence in units of the determined expansion size;
An optical communication method comprising:

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